Universal Time

The Modified Julian Date is the day with the fraction of the day, measured from UT midnight.
It's used to represent UT1, which is time as measured by the earth's rotation, adjusted for various wobbles.

UTC

UTC is time as measured by a clock, corrected to keep pace with the earth by adding or removing
occasional seconds, known as "leap seconds".
These corrections are not predictable and are announced with six month's notice.
No table of these corrections is provided, as any program compiled with it would become
out of date in six months.

If you don't care about leap seconds, use UTCTime and NominalDiffTime for your clock calculations,
and you'll be fine.

This is a length of time, as measured by UTC.
Conversion functions will treat it as seconds.
It has a precision of 10^-12 s.
It ignores leap-seconds, so it's not necessarily a fixed amount of clock time.
For instance, 23:00 UTC + 2 hours of NominalDiffTime = 01:00 UTC (+ 1 day),
regardless of whether a leap-second intervened.